Genetics and Evolution
Genetics and Evolution examines inheritance patterns, the molecular flow of information, and the mechanisms that change populations over time. The exam often combines vocabulary with numerical problems and experimental deductions.
Core rules and concepts
- Mendel's law of segregation explains why alleles separate in gamete formation; independent assortment applies to genes on different chromosomes. Incomplete dominance gives a 1:2:1 phenotypic ratio in the F2, while codominance is shown by ABO alleles IA and IB.
- DNA is antiparallel and double-stranded, with A pairing with T and G with C. Replication is semiconservative. Transcription makes RNA, and the genetic code is triplet and degenerate.
- The lac operon is inducible: lactose inactivates the repressor, allowing transcription of lacZ, lacY, and lacA. Evolution involves natural selection, genetic drift, mutation, gene flow, and speciation.
Worked example
In a population at Hardy-Weinberg equilibrium, the recessive allele frequency is q = 0.2. Then p = 1 - 0.2 = 0.8. Heterozygote frequency is 2pq = 2 x 0.8 x 0.2 = 0.32, or 32%. The reasoning depends on using q for the recessive allele frequency, not for the recessive genotype frequency.
Traps that cost marks
Dominance does not mean blending; heterozygotes retain both alleles. Independent assortment does not apply to linked genes. DNA polymerase adds nucleotides at the 3' end, so synthesis proceeds 5' to 3'. In Hardy-Weinberg problems, q² is the recessive genotype frequency, while q is the allele frequency.